The water that is purified in municipal sewage works is collected from a large number of sources. It usually comprises the accumulated flows of domestic wastewater, surface water, partly purified industrial wastewater and rain water. In wastewater purification, first large objects that have got into the wastewater are removed. Moreover removal is made of particles and gravel, and finally organic matter, metals and nutrient salts, such as nitrogen and phosphorus compounds.
Normal wastewater purification in Sweden comprises mechanical, biological and chemical purification of the water. A grating separates the coarsest impurities, after which sand and heavier particles settle in a first settling step, referred to as a sand trap, before the chemical and/or biological purification starts.
Today's Swedish sewage works are built with extensive biological and chemical purification. Metal salts of aluminium or iron are used to precipitate phosphate. They can be added before, to or after the biological purification process (preprecipitation, simultaneous precipitation and postprecipitation respectively). Both the chemical and the biological treatment of the water generate sludge which settles and must be taken care of.
The generated sludges from the different sedimentations are usually collected in the sewage works involved. It is then usually a matter of primary sludge (mechanical sludge), secondary sludge (biological excess sludge) and tertiary sludge (chemical sludge). Sedimentation products from initial gratings and sand traps are usually not included in the thus collected sludge. The collected sludge is slightly thickened by additional settling and is then pumped into a digester. In the digester, the sludge is treated under anaerobic conditions for breakdown of organic matter and production of a reduced amount of sludge, referred to as digested sludge.
In wastewater purification as described above, large amounts of digested sludge are obtained, which must be taken care of. The digested sludge can, for instance, be deposited on landfill sites or be used as fertiliser. Depositing the digested sludge on landfill sites requires, however, large spaces and is costly. Use of digested sludge as fertiliser is preferred, but this use has been increasingly questioned since the sludge contains heavy metals and other undesirable substances. As an alternative to depositing on landfill sites and use as fertiliser, the digested sludge can be burnt. In Sweden, depositing of combustible waste on landfill sites was prohibited in 2002, and in 2005 depositing of all organic waste on landfill sites will be prohibited.
In view of that stated above, digested sludge that is to be incinerated should have as high a solids content as possible. Also in the case of another type of final storage of the dewatered sludge than incineration, it can be very important, above all in economical terms, to reduce the amount of sludge by dewatering the sludge to as high a solids content as possible. Reducing the amount of sludge by providing a high solids content of the digested sludge is difficult and has so far been achieved only at a high cost by a combination of mechanical dewatering and drying. It would therefore constitute a great improvement if a reduction of the amount of digested sludge, i.e. the weight and volume of the digested sludge, could easily be provided in a way other than by dewatering and drying only.
A further problem in treatment of digested sludge from wastewater purification is the disturbing and unpleasant smell that is associated with the digested sludge. This disturbing smell is an environmental problem and implies that plants for production and treatment of sludge from wastewater purification must often be located isolated from other buildings. It would therefore constitute an environmental advantage if the disturbing and unpleasant smell from the sludge could be reduced or eliminated.
A further problem is that sludge from wastewater purification often contains pathogenic bacteria, such as salmonella, E. Coli etc. It would be a great advantage if the sludge could be sanitised, i.e. treated so that such pathogenic bacteria are eliminated or reduced to a harmless level (for instance below 10 cfu/ml sludge with a solids content of 1–5%. “cfu”=colony forming units).
Yet another problem is that sludge from wastewater purification is in most cases sticky and difficult to handle and dewater. It would imply a considerable advantage if a sludge could be provided that is not sticky and is easy to handle and dewater.
Different methods for treatment of sludge from wastewater purification are known, and as an example mention can be made of WO 95/06004, which was published on 2 Mar. 1995, and WO 96/20894, which was published on 11 Jul. 1996. These two references concern treatment of sludge from wastewater purification for recovery of phosphorus and metal, for instance iron, from the precipitating chemical. The sludge can be treated with acid for dissolution of the metal and phosphorus content, after which phosphorus is precipitated as trivalent iron phosphate, in which case divalent iron can first be converted to trivalent iron by oxidisation with, for instance, hydrogen peroxide.
As a further example, mention can be of WO 98/41479, which was published on 24 Sep. 1998. This reference discloses a method for treatment of sludge from waste-water purification, in which iron and/or aluminium from the precipitating chemicals is dissolved from the sludge and the formed solution is recirculated to the wastewater purification. In a first step, the sludge is subjected to acid hydrolysis. After hydrolysis, the remaining sludge and hydrolysis liquid are fed to a second step for separation of the remaining sludge.
A further example of prior-art technique regarding sludge treatment is EP 0 832 853. This reference concerns a method of removing undesirable smell from sludge from biological wastewater purification and improving the filterability of the sludge. In the method, sludge at a pH of 2–6 is mixed with an iron(II)salt and hydrogen peroxide. pH is adjusted using an acid, such as sulphuric acid, which suitably is added simultaneously with the iron(II)salt. An exothermal reaction is obtained, which generates a temperature of 10–38° C. of the reaction mixture, without heating being required.
One more example of prior-art technique regarding sludge treatment is U.S. Pat. No. 6,368,511 which discloses a method for improving the dewatering of sludge. In the method, the sludge is subjected to an acid oxidative pre-conditioning at a pH below 5. Acidification occurs with hydrochloric acid to prevent subsequent problems with precipitation of gypsum, as would be the case in acidification using sulphuric acid. In the preconditioning, divalent iron ions and hydrogen peroxide are added, thereby forming Fenton's reagents which cause a partial oxidative breakdown of organic sludge components. Then an inorganic postconditioning is carried out by the preconditioned sludge being mixed with alkaline earth metal oxides, such as calcium hydroxide, in order to increase the pH to 9–11. Subsequently the sludge is dewatered.
Journal of Hazardous Materials B98 (2003) 33–50, “A Review of Classic Fenton's Peroxidation as an Advanced Oxidation Technique” provides a survey of Fenton's reaction. It also describes use thereof to improve the dewatering capacity of sewage sludge, However, no particular acid treatment of the sludge is described. Nor is the phosphorus content of the sludge stated, nor that this is precipitated as trivalent iron phosphate.
Journal of Hazardous Materials B98 (2003), 91–106 “Pilot-Scale Peroxidation (H2O2) of Sewage Sludge” describes treatment of sewage sludge on a pilot scale by Fenton's reaction in order to improve the dewatering capacity. According to the article, optimal conditions are pH 3, addition of 5–50 g H2O2/kg solid matter, 1.67 Fe2+/kg solid matter at ambient temperature and pressure for 60–90 min. After reaction, neutralisation was carried out by adding calcium hydroxide. A special acid treatment of the sludge is not described. Nor is it stated that the phosphorus content of the sludge is precipitated as trivalent iron phosphate. Instead it is evident that the dissolution of phosphorus to the aqueous phase has increased, and also that the dissolution of nitrogen has increased significantly.
Patent Abstracts of Japan, Vol. 006, No. 063 and JP 57,004,299 (EBARA INFILCO CO LTD), 1982, describe a method for sludge treatment, in which first an oxidiser, such as hydrogen peroxide, and a metal ion dissociating material, such as ferrosulphate, are added to the sludge. Subsequently the sludge is treated under acid conditions, for instance with sulphuric acid, after which the treated sludge is dewatered.
WO 03/045851, which was published on 5 Jun. 2003, discloses a method for treatment of sludge from waste-water purification, the sludge being acidified in two steps and treated with a solution of ferrisalt and hydrogen peroxide in an amount 5–40 kg Fe/tonne of dry sludge and respectively 5–40 kg H2O2/tonne of dry sludge. The sludge is flocculated by adding an organic polymer, after which the flocculated sludge is dewatered.
WO 03/045852, which was published on 5 Jun. 2003, comprises the same method as WO 03/045851, except that acidification preferably occurs in one and the same step.
An object of the present invention is to provide a method of treating sludge from wastewater purification in order to easily and effectively reduce the amount thereof, i.e. weight and/or volume thereof.
Another object of the invention is to provide a method of treating sludge, by which the sludge is deodorised, i.e. the disturbing and unpleasant smell is removed from the sludge.
One more object of the invention is to provide a method of treating sludge, by which the sludge is sanitised.
A further object of the invention is to provide a method of treating sludge, which results in a sludge with improved dewatering capacity, i.e. a sludge that is easier and/or quicker to dewater;